1. Field of the Invention
The present invention relates to a bridge circuit and method for testing for MR (magnetoresistive) shield short circuits and more particularly to a bridge circuit and method for testing for short circuits between lead and shield layers of a thin film MR read head that does not require direct electrical contact to the shield.
2. Description of the Related Art
A thin film MR head employs a magnetoresistive (MR) sensor for reading information signals from a moving magnetic medium, such as a rotating magnetic disk. First and second leads are employed for connecting the MR sensor to first and second externally located read pads. A sense current is applied to the MR sensor via the read pads. Magnetic fields from the magnetic medium cause resistance changes in the MR sensor. The resistance changes cause potential changes across the read pads which are sensed by a detector, such as a differential amplifier. The output of the detector is the readback signal.
The shields prevent destructive interference of the magnetic field from adjacent transitions in the media (a.k.a. ISI or intersymbol interference). Accordingly, the MR sensor and the first and second leads are sandwiched between first and second insulation gap layers and the first and second gap layers are sandwiched between first and second shield layers. The MR head, which comprises the MR sensor, literally "flies" with respect to a rotating magnetic disk, supported by a thin cushion of air that the disk moves between the disk and the ABS. The thin cushion of air is commonly referred to as an "air bearing." The air bearing provides a first force which is counterbalanced by a second force from a suspension which carries the MR head. The counterbalance can be designed to provide a very small distance ("flying height") between the ABS and the rotating disk, such as 0.075 microns. With this arrangement the MR head can read information from each circular track on a rotating magnetic disk with high resolution.
It is important for high linear density that the MR sensor have good resolution. Resolution is the sharpness of the reading capability of the MR head along the length of a circular track on the rotating magnetic disk. Resolution is dependent upon the flying height and the thickness of the first and second gap layers which separate the MR sensor from the first and second shield layers. Production MR heads typically have very thin first and second gap layers, typically less than 0.5 microns. Because of the thinness of the gap layers there are occasional openings through the gap layers which are referred to as "pinholes". A pinhole or other types of defects can cause an internal shield short between a lead and a shield. If the first gap layer has a pinhole, a shield short can occur between one of the leads and the first shield layer and if the second gap layer has a pinhole a shield short can occur between one of the leads and the second shield layer. Internal shield shorts increase noise pickup and can reduce the amplitude if sense current is shunted to the shields. It is desirable that internal shield shorts be detected during production so that defective heads can be discarded.
An MR head can be used singly or in combination with a write head. Typically a thin film MR head is combined with a thin film write head which has first and second pole pieces that define a gap, the pole pieces terminating in pole tips at the ABS. To write information, magnetic fields are induced into the gap by a coil which is embedded in an insulation stack between the first and second pole pieces and which is a signal current through first and second externally located write pads. A combined MR read head and write head is constructed as a merged MR head or a piggyback MR head. A merged MR head employs the second shield layer as the first pole piece, while the piggyback MR head has a separate second shield layer and a separate first pole piece.
Thin film MR read heads, per se, merged MR heads or piggyback MR heads are typically constructed in rows and columns on a wafer. During wafer construction all of the thin film layers are formed for the heads. The wafer is then separated into rows so that a plurality of heads are aligned in a single row. At the row level the heads are lapped to form the ABS. Lapping can sometimes cause a smearing of the conductive shields between the shields and the MR sensor or leads at the ABS. Such smearing also causes an ABS shield short between the MR circuit and one or more of the shields with the same problems as mentioned hereinabove with respect to pinholes. It would be highly desirable to detect shield shorts at the row level which are caused by pinholes in the gap layers or conductive smearing at the ABS. After testing the row can be diced into individual heads. A portion of the wafer remains and provides an externally located substrate and/or slider for supporting the completed head. It would also be desirable to be able to detect a shield short in a completed head which is mounted on a head gimbal assembly (HGA) for use in a drive, such as a magnetic disk drive.
In the past, extra pads, extra probes and changes in head layout have been employed to accommodate testing for head short circuits during production. As an example holes (vias) have been made in the layers to the shield layers so that external pads can be provided for external testing equipment. The pads can take up extra room on the wafer thus reducing production throughput. Also, there may not be room on the slider which is getting smaller as technology advances. It would be desirable if direct contact with the shield layers could be eliminated when testing for shield short circuits.